Which AWS service is described as 'serverless' and offers automatic scalability?

The AWS service that is 'serverless' and offers automatic scalability is AWS Glue. This service provides an easy way to prepare and transform data for analytics without the need to manage any infrastructure. By being serverless, AWS Glue abstracts away the underlying servers, allowing users to focus solely on their data processing tasks.

AWS Glue automatically scales to match the workload, meaning it can handle varying amounts of data without needing user intervention to provision or manage server resources. This feature allows for efficient utilization of resources and can lead to cost savings, as users only pay for the resources consumed during their data processing jobs.

While Amazon EMR is a powerful service for big data processing, it requires users to manage the underlying clusters, which makes it less serverless. Amazon S3 is primarily an object storage service and does not inherently have serverless processing capabilities. Similarly, Amazon CloudSearch is a search service that requires infrastructure to be managed, which does not align with the serverless model.

Therefore, AWS Glue stands out as the optimal choice for users looking for serverless and automatically scalable data processing solutions.